1. Field of the Invention
The present invention relates generally to illumination systems that can be used in lithography tools.
2. Background Art
Lithography can be used to fabricate patterns on substrates, semiconductor wafers, flat panel displays (glass substrates), and like. For ease of explanation, the description in the specification will be based on semiconductor fabrication. In lithography, various wavelengths of light are used, depending on a size of a feature being pattered, to image patterns onto the semiconductor wafer. Many types of semiconductor devices, such as diodes, transistors, and integrated circuits, can be fabricated using lithographic techniques.
A lithography system typically includes an illumination system that illuminates a pattern generator containing a pattern (e.g., a reticle (also called a mask), a spatial light modulator or contrast device (a digital mirror device, a grating light valve, a liquid crystal display), or the like, hereinafter all are referred to as pattern generator) to image the pattern, via a projection system, on a semiconductor wafer in an exposure system. Typically, the illumination system illuminates a region of the pattern generator and the projection system projects an image of the illuminated region onto the wafer.
As semiconductor device manufacturing technology advances, there are ever increasing demands on each component of the lithography system used to manufacture the semiconductor device. This includes the illumination system used to illuminate the pattern generator. For example, there is a need to illuminate the pattern generator with an illumination field having uniform irradiance. In step-and-scan lithography, there is also a need to continuously vary a size of the illumination field in a direction perpendicular to a wafer scan direction, so that the size of the illumination field can be tailored to different applications. One factor often limiting wafer processing throughput is the amount of energy available from the illumination system. As a result, there is a need to vary the size of the illumination field without a loss of energy.
As the size of the illumination field is varied, it is important to preserve the angular distribution and characteristics of the illumination field at the pattern generator. To achieve this goal, the illumination system must maintain telecentric illumination at a substantially fixed numerical aperture at the pattern generator as the size of the illumination field is varied. Some illumination systems include a scattering optical element, such as an array, positioned before the pattern generator. The scattering optical element produces a desired angular light distribution that is subsequently imaged or relayed to the pattern generator. In such an illumination system, there is a need to maintain telecentric illumination at a substantially fixed numerical aperture at the scattering optical element, and correspondingly, at the pattern generator, as the size of the illumination field is varied.
A standard zoom lens can vary the size of the illumination field. However, in the standard zoom lens, image magnification, and correspondingly the size of the illumination field, is inversely proportional to angular magnification. Thus, a standard zoom lens that increases the size of an image by a factor M, disadvantageously decreases the numerical aperture by a factor 1/M, and fails to preserve the angular distribution of the illumination field.
In conventional systems, only one of a field height or pupil can be varied, while maintaining constant radiometric efficiency. Thus, in systems that have the capability to continuous vary a field height, hard apertures are inserted to create fixed pupils, which reduces radiometric efficiency. Similarly, systems that have continuously varying pupils require hard apertures to reduce the field height, which again reduces radiometric efficiency.
Therefore, what is needed is a system and method that allow for varying both a field height and pupil of an illumination beam, possibly continuously (dynamically), while desirably maintaining radiometric efficiency of a system.